Multi-alloy composite sheet for automotive panels

ABSTRACT

Multi-alloy composite sheets and methods of producing the composite sheets for use in automotive applications are disclosed. The automotive application may include an automotive panel having a bi-layer or a tri-layer composite sheet with 3xxx and 6xxx aluminum alloys. The composite sheets may be produced by roll bonding or multi-alloy casting, among other techniques. Each of the composite sheets may demonstrate good flat hem rating and mechanical properties, long shelf life, and high dent resistance, among other properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application No.12/768,429, filed Apr. 27, 2010, which claims priority of U.S.Provisional Patent Application Ser. No. 61/174,324 filed Apr. 30, 2009,each of which are incorporated herein by reference in their entirety forall purposes.

BACKGROUND

Automotive panels generally include an outer panel and an inner panel.These outer and inner panels must achieve certain properties. Forexample, an outer panel generally must meet adequate flat hem rating,after paint-bake strength for dent resistance, class A painted surfacequality, and overall good formability, among other factors. Anotherhighly desirable aspect for hemming performance is that the material beimmune to natural aging. For an inner panel, it generally must meettypically higher formability measured by adequate limiting dome heightor limiting draw ratio.

SUMMARY

Automotive panels fabricated of a multi-alloy composite sheet and methodof producing the same are disclosed. In one embodiment, a multi-alloycomposite sheet includes an Al—Mg—Si alloy layer and an Al—Mg alloylayer coupled to at least one surface of the Al—Mg—Si alloy layer. Insome embodiments, the resulting composite sheet is capable of achievinga flat hem rating of not worse than 3, or not worse than 2, or not worsethan 1.

In one embodiment, the Al—Mg—Si alloy is a 6xxx series aluminum alloyand the Al—Mg alloy is a 3xxx series aluminum alloy. In anotherembodiment, the Al—Mg—Si alloy layer has a thickness in the range offrom about 60% to about 90% of the total thickness of the compositesheet, while the Al—Mg alloy layer has a thickness in the range of fromabout 10% to about 40% of the total thickness of the composite sheet.

In some embodiments, the flat hem rating is measured at a pre-strainlevel of at least about 1%, or at least about 7%, or at least about 11%,or at least about 15%. In other embodiments, the flat hem rating ismeasured at a time period of at least about 7 days, or at least about 14days, or at least about 30 days, or at least about 60 days, or at leastabout 90 days.

In some embodiments, the composite sheet is capable of achieving a yieldstrength of at least about 190 MPa after a paint bake cycle, or at leastabout 210 MPa, or at least about 230 MPa. In other embodiments, thecomposite sheet is capable of achieving a limiting dome height of atleast about 20 mm, or at least about 22 mm, or at least about 24 mm.

In one embodiment, a multi-alloy composite sheet includes an Al—Mg—Sialloy layer and two Al—Mg alloy layers where the first Al—Mg alloy layeris coupled to one surface of the Al—Mg—Si alloy layer and the secondAl—Mg alloy layer is coupled to another surface of the Al—Mg—Si alloylayer, the two surfaces being opposite each other. The resultingcomposite sheet is capable of achieving a flat hem rating of not worsethan 3, or not worse than 2, or not worse than 1.

In one embodiment, the Al—Mg—Si alloy is a 6xxx series aluminum alloyand each of the two Al—Mg alloys is a 3xxx series aluminum alloy. Inanother embodiment, the Al—Mg—Si alloy layer has a thickness in therange of from about 50% to about 80% of the total thickness of thecomposite sheet, the first Al—Mg alloy layer has a thickness in therange of from about 10% to about 40% of the total thickness of thecomposite sheet, and the second Al—Mg alloy layer has a thickness in therange of from about 0% to about 10% of the total thickness of thecomposite sheet.

In some embodiments, the flat hem rating is measured at a pre-strainlevel of at least about 1%, or at least about 7%, or at least about 11%,or at least about 15%. In other embodiments, the flat hem rating ismeasured at a time period of at least about 7 days, or at least about 14days, or at least about 30 days, or at least about 60 days, or at leastabout 90 days. In one embodiment, the composite sheet is capable ofachieving a yield strength of at least about 190 MPa after a paint bakecycle.

In one embodiment, a method of producing at least a bi-layer compositesheet includes producing an Al—Mg—Si alloy layer, an Al—Mg alloy layer,and placing the two alloy layers in physical contact with each othersuch that the resulting composite sheet achieves a flat hem rating ofnot worse than 3, or not worse than 2, or not worse than 1. In someembodiments, the method of producing the composite may be carried out byat least one of roll bonding, multi-alloy casting and direct chillcasting. In other embodiments, the Al—Mg—Si alloy is a 6xxx seriesaluminum alloy and the Al—Mg alloy is a 3xxx series aluminum alloy.

In another embodiment, a method of producing at least a tri-layercomposite sheet includes producing a second Al—Mg alloy layer inaddition to the first Al—Mg alloy layer from above, and placing thesecond Al—Mg alloy layer in physical contact with a surface of theAl—Mg—Si alloy layer opposite that of the first Al—Mg alloy layer. Theresulting tri-layer composite sheet is capable of achieving a flat hemrating of not worse than 3, or not worse than 2, or not worse than 1.Like above, in some embodiments, the method of producing the tri-layercomposite sheet includes at least one of roll bonding, multi-alloycasting and direct chill casting. Furthermore, the second Al—Mg alloy,like the first Al—Mg alloy, may also be a 3xxx series aluminum alloy.

Other variations, embodiments and features of the present disclosurewill become evident from the following detailed description, drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of an automotive body.

FIG. 2 shows an exploded view of a car hood.

FIG. 3 shows cross-section views of composite sheets with various rangeof thicknesses and layer percentages.

FIG. 4 is a process flow diagram showing the various steps of producinga composite sheet according to one embodiment of the present disclosure.

FIG. 5 is a process flow diagram showing the various steps of producinga composite sheet according to one embodiment of the present disclosure.

FIG. 6 is a process flow of a hemming process.

FIG. 7 shows cross-sectional views of different types of hemming.

FIG. 8 illustrates standards for determining flat hem ratings of testspecimens.

FIG. 9 shows optical micrographs of hemming test specimens.

FIG. 10 is a dog-boned shaped standard tensile test specimen.

FIG. 11 is a process flow diagram showing the various steps ofmanufacturing a composite sheet according to one embodiment of thepresent disclosure.

FIG. 12 illustrates cross-sectional optical micrographs of hemming sitesfor a 6xxx aluminum alloy after 3 months natural aging.

FIG. 13 illustrates cross-sectional optical micrographs of hemming sitesfor a tri-layer composite sheet according to one embodiment of thepresent disclosure after 3 months natural aging.

DETAILED DESCRIPTION OF THE DISCLOSURE

It will be appreciated by those of ordinary skill in the art that theembodiments disclosed herein can be embodied in other specific formswithout departing from the spirit or essential character thereof. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restrictive.

Multi-alloy composite sheets and methods of producing the same forautomotive applications are disclosed. In general, the multi-alloycomposite sheets are capable of achieving enhanced attributes overcurrent products on the market. Specifically, the presently disclosedmulti-alloy composite sheets may demonstrate better hem performance,longer shelf life, higher after paint-bake strength, better paintedsurface quality, higher formability and better corrosion resistance,among other characteristics.

The instant application relates to automotive panels generally having anouter panel and an inner panel, where each of the outer panel and theinner panel is capable of achieving certain properties. In oneembodiment, each of the outer panel and the inner panel may be formed ofa multi-alloy composite sheet.

As used herein, “panel” means a sheet that forms a distinct, sometimesflat, section or component of something. As used herein, “sheet” meansan artifact that is thin relative to its length and width. Examples ofsheets include panels, such as automotive panels, which may be in theform of composite sheets. As used herein, “automotive panel” means apanel for automotive applications including the likes of hoods, fenders,doors, roofs, and trunk lids, among others.

Reference is now made to FIG. 1 showing an exploded view of anautomotive body 100 having a plurality of automotive panels 110. In oneembodiment, an automotive panel 110 includes the likes of car hoods 110a, car fenders 110 b, car doors 110 c, car roofs 110 d, and trunk lids110 e, among others. In some embodiments, an automotive panel 110 mayinclude closure panels and fender liners, among other parts of anautomobile. In general, an automotive panel 110 may form a distinctportion of an automobile.

In one embodiment, an automotive panel 110 includes an outer panel andan inner panel. The outer panel includes, in one embodiment, at leastone composite sheet, as described further below. Similarly, in anotherembodiment, an inner panel may also include at least one compositesheet, where the composite sheet of the inner panel need not be the sameas the composite sheet of the outer panel. Generally speaking, an outerpanel is the portion of an automotive panel 110 that is intended to beexposed to outdoor conditions, while an inner panel is the portion of anautomotive panel 110 that is not intended to be exposed to outdoorconditions.

Reference is now made to FIG. 2 showing an exploded view of a car hood110 a having an outer panel 210 and an inner panel 230. An inset of theouter panel 210 shows a cross-sectional view of the outer panel 210having a first composite sheet 220, the first composite sheet 220 havinga first outer layer 220 a and a first inner layer 220 b. Likewise, aninset of the inner panel 230 shows a cross-sectional view of the innerpanel 230 having a second composite sheet 240, the second compositesheet 240 having a second outer layer 240 a and a second inner layer 240b. In some instances, the outer layers 220 a, 240 a may be referred toas skins or skin layers, while the inner layers 220 b, 240 b may bereferred to as cores or core layers.

As used herein, “composite sheet” means a sheet having at least twodistinct layers, such as an inner layer (e.g., core layer) and an outerlayer (e.g., skin layer). For example, an automotive panel may include acomposite sheet 220, 240 having at least an outer layer 220 a, 240 a andan inner layer 220 b, 240 b, where the outer layer 220 a, 240 a isfabricated of a first material (e.g., a first aluminum alloy) and theinner layer 220 b, 240 b is fabricated of a second material (e.g., asecond aluminum alloy). The layers 220 a, 220 b, 240 a, 240 b of thecomposite sheet 220, 240 may be produced via multi-alloy casting, directchill (DC) casting, and roll bonding, among other suitable techniques.In some embodiments, the outer layer 220 a, 240 a and the inner layer220 b, 240 b may be coupled to each other via metallurgical bonding.

Like the outer and inner automotive panels, an outer layer of acomposite sheet means a layer generally intended to be exposed tooutdoor conditions while an inner layer of a composite sheet means alayer generally not intended to be exposed to outdoor conditions. Insome instances, the outer layer may be referred to as a skin materialwhile an inner layer may be referred to as a core material, such as whenin use in an automotive panel.

In one embodiment, an Al—Mg—Si alloy may be suitable as an outer layer220 a, 240 a of a composite sheet 220, 240. In one embodiment, anAl—Mg—Si alloy may be suitable as an inner layer 220 b, 240 b of acomposite sheet 220, 240. As used herein, “Al—Mg—Si alloy” means analuminum alloy having magnesium and silicon as primary alloyingconstituents. In some instances, the Al—Mg—Si alloy may also containalloying additions including copper, chromium, titanium, manganese,zinc, iron, silicon and vanadium, among others.

In one embodiment, the Al—Mg—Si alloy is AA6013. As used herein,“AA6013” means Aluminum Association alloy 6013, as defined by theAluminum Association Teal Sheets. Examples of Al—Mg—Si alloys includeany of the 6xxx series alloys including at least one of AA6022, AA6111,AA6061, AA6063, AA6016, AA6056, AA6082, AA6181 and AA6181A, amongothers, as defined by the Aluminum Association Teal Sheets.

In one embodiment, an Al—Mg alloy may be suitable as an outer layer 220a, 240 a of a composite sheet 220, 240. In one embodiment, an Al—Mgalloy may be suitable as an inner layer 220 b, 240 b of a compositesheet 220, 240. In some embodiments, an Al—Mg alloy may be suitable asboth outer layer 220 a, 240 a and inner layer 220 b, 240 b of acomposite sheet 220, 240. In other words, a composite sheet may includeat least three distinct layers with a first Al—Mg alloy as an outerlayer and a second Al—Mg alloy as an inner layer, the two Al—Mg alloylayers coupled on opposite surfaces of an Al—Mg—Si alloy layer. The twoAl—Mg alloy layers may have the same or different compositions.

In other embodiments, a composite sheet may include multiple distinctlayers with various combinations of Al—Mg—Si and Al—Mg alloys. As usedherein, “Al—Mg alloy” means an aluminum alloy having manganese as aprimary alloying constituent. In some embodiments, the Al—Mn alloy mayalso contain alloying additions including manganese, copper, chromium,iron, silicon and titanium, among others.

In one embodiment, the Al—Mg alloy is AA3104. As used herein, “AA3104”means Aluminum Association alloy 3104, as defined by the AluminumAssociation Teal Sheets. In another embodiment, the Al—Mg alloy isAA3003. As used herein, “AA3003” means Aluminum Association alloy 3003,as defined by the Aluminum Association Teal Sheets. Examples of Al—Mgalloys include any of the 3xxx series alloys including at least one ofAA3004 and AA3005, among others, as defined by the Aluminum AssociationTeal Sheets.

Reference is now made to FIG. 3 showing cross-sectional views ofcomposite sheets 320, 360 with various ranges of thicknesses and layerpercentages. As shown, in one embodiment, a composite sheet 320 includesan Al—Mg—Si alloy core layer 340 coupled to an Al—Mg alloy skin layer330. As described above, the layers 330, 340 may be coupled to eachother by at least one of roll bonding, multi-alloy casting and directchill casting. In one example, the Al—Mg—Si alloy core layer 340 may beAA6013 and the Al—Mg alloy skin layer 330 may be AA3003. Alternatively,the Al—Mg—Si alloy core layer 340 may be AA6013 and the Al—Mg alloy skinlayer 330 may be AA3104. In some instances, the Al—Mg—Si alloy corelayer 340 includes one of 6xxx series aluminum alloys and the Al—Mgalloy skin layer 330 includes one of 3xxx series aluminum alloys.

Also shown, in one embodiment, a composite sheet 360 includes a firstAl—Mg alloy skin layer 370 coupled to a first surface of an Al—Mg—Sialloy core layer 380, and a second Al—Mg alloy skin layer 390 coupled toa second surface of the Al—Mg—Si alloy core layer 380, where the firstsurface is opposite the second surface. Like above, the layers 370, 380,390 may be coupled to each other by at least one of roll bonding,multi-alloy casting and direct chill casting. In one example, theAl—Mg—Si alloy core layer 380 may be AA6013 and the Al—Mg alloy skinlayers 370, 390 may be AA3003. Alternatively, the Al—Mg—Si alloy corelayer 380 may be AA6013 and the Al—Mg alloy skin layers 370, 390 may beAA3104. In some instances, the Al—Mg—Si alloy core layer 380 includesone of 6xxx series aluminum alloys and the Al—Mg alloy skin layers 370,390 include one of 3xxx series aluminum alloys.

In some embodiments, the total thickness T1, T2 of the composite sheets320, 360 may be at least about 0.1 mm, or at least about 0.2 mm, or atleast about 0.3 mm, or at least about 0.4 mm, or at least about 0.5 mm,or at least about 1 mm, or at least about 2 mm, or at least about 3 mm,or at least about 4 mm, or at least about 5 mm, or at least about 6 mm,or at least about 8 mm, or at least about 10 mm, or at least about 15mm. In other embodiments, the total thickness T1, T2 of the compositesheets 320, 360 may be in the range of from about 0.1 mm to about 15 mm,or from about 0.5 mm to about 5 mm, or from about 1 mm to about 2 mm.

In one embodiment, the composite sheets 320, 360 may be produced in a T4condition with a total thickness T1, T2 of about 1 mm, where T4condition means a material that is solution heat treated and naturallyaged. Solution heat treatment and natural aging will become moreapparent in subsequent discussion below.

In one embodiment, the Al—Mg—Si alloy core layer 340 may be in the rangeof from about 60% to about 90% of the total thickness T1 of thecomposite sheet 320 while the Al—Mg alloy skin layer 330 may be in therange of from about 10% to about 40% of the total thickness T1 of thecomposite sheet 320. In the alternative, the Al—Mg—Si alloy core layer340 and the Al—Mg alloy skin layer 330 may include other suitablethickness ranges.

In one example, the total thickness T1 of the composite sheet 320 may beabout 1 mm with the Al—Mg alloy skin layer 330 at about 0.15 mm and theAl—Mg—Si alloy core layer 340 at about 0.85 mm. In another example, thetotal thickness T1 of the composite sheet 320 may be about 1 mm with theAl—Mg alloy skin layer 330 at about 0.2 mm and the Al—Mg—Si alloy corelayer 340 at about 0.8 mm. In some instances, the total thickness T1 ofthe composite sheet 320 may be about 1 mm with the Al—Mg alloy skinlayer 330 at about 0.25 mm and the Al—Mg—Si alloy core layer 340 atabout 0.75 mm. In other instances, the total thickness T1 of thecomposite sheet 320 may be about 1 mm with the Al—Mg alloy skin layer330 at about 0.3 mm and the Al—Mg—Si alloy core layer 340 at about 0.7mm. In one example, the total thickness T1 of the composite sheet 320may be about 6 mm with the Al—Mg alloy skin layer 330 at about 1.5 mmand the Al—Mg—Si alloy core layer 340 at about 4.5 mm.

In one embodiment, the Al—Mg—Si alloy core layer 380 may be in the rangeof from about 50% to about 80% of the total thickness T2 of thecomposite sheet 360, the first Al—Mg alloy skin layer 370 may be in therange of from about 10% to about 40% of the total thickness T2 of thecomposite sheet 360, and the second Al—Mg alloy skin layer 390 may be inthe range of from about 0% to about 10% of the total thickness T2 of thecomposite sheet 360. In another embodiment, the Al—Mg—Si alloy corelayer 380 may be in the range of from about 20% to about 80% of thetotal thickness T2 of the composite sheet 360 and each of the Al—Mgalloy skin layers 370, 390 may be in the range of from about 10% toabout 40% of the total thickness T2 of the composite sheet 360 (notshown). In one embodiment, the first Al—Mg alloy skin layer 370 may bemore suitable as an outer layer while the second Al—Mg alloy skin layer390 may be more suitable as an inner layer. In the alternative, theAl—Mg—Si alloy core layer 380 and the Al—Mg alloy skin layers 370, 390may include other suitable thickness ranges. In one example, the totalthickness T2 of the composite sheet 360 may be about 1 mm with the firstAl—Mg alloy skin layer 370 at about 0.25 mm, the Al—Mg—Si alloy corelayer 380 at about 0.65 mm, and the second Al—Mg alloy skin layer 390 atabout 0.1 mm.

Reference is now made to FIG. 4 illustrating a process flow diagram 400of the various steps of producing a composite sheet according to oneembodiment of the present disclosure. In one embodiment, an ingot havingat least one aluminum alloy may be produced by a casting step 410. Thecasting step 410 includes casting aluminum alloy ingots via multi-alloycasting or DC casting, among other suitable casting techniques. Forexample, a monolithic ingot (e.g., single layer) may be produced bycasting a single aluminum alloy material. In other examples, a compositeingot (e.g., multi-alloy) may be produced by casting at least twoaluminum alloys, where each aluminum alloy has a different chemicalcomposition. In one embodiment, a composite ingot may be produced byseparately casting at least two different aluminum alloy layers, andsubsequently bringing and placing the aluminum alloy layers in physicalcontact with one another to from the composite ingot.

After the casting step 410, the monolithic or composite ingot may besubjected to a homogenizing step 420. In one embodiment, thehomogenizing step 420 includes heating the ingot at temperatures rangingfrom about 540° C. to about 570° C. for about 4 hours. The homogenizingstep 420 allows diffusion of species or other elements (e.g., magnesium,silicon) within the composite sheet. In some instances, the homogenizingstep 420 may remove micro-segregations and enhance ingot uniformity.

The thickness of the ingot may subsequently be reduced to a desiredgauge (e.g., sheet thickness) by a hot rolling step 430. In general, thehot rolling step 430 involves the use of heavy mechanical rollers thatapply pressure to flatten or reduce the thickness of the ingot. Combinedwith high temperatures, the hot rolling step 430 may reduce thethickness of the ingot to the desired sheet thickness ranges renderingthe sheet more suitable for subsequent processing steps. For example, aningot having a thickness of about 304.8 mm (12 inches) may be hot rolledto a sheet having a thickness of about 3.4 mm (0.135 inch). At thebeginning of the hot rolling step 430, the ingot may be at a temperaturein the range of from about 500° C. to about 550° C. And at theconclusion of the hot rolling step 430, the sheet may be maintained at atemperature in the range of from about 250° C. to about 350° C. Thecombination of pressure from the mechanical rollers and the highertemperature may facilitate a nearly 10-fold reduction in ingot thicknessto produced a monolithic or composite sheet with a thickness of notgreater than about 15 mm or about 10 mm. In some instances, the sheetmay be wound into a coil or unwound into sheet during the hot rollingstep 430.

The monolithic or composite sheet may subsequently be subjected to athermal processing step 440. In one embodiment, the thermal processingstep 440 includes batch annealing (BA) the sheet at a temperature in therange of from about 420° C. to about 430° C. for about 60 minutes. Inanother embodiment, the thermal processing step 440 includes solutionheat treatment (SHT) of the sheet at a temperature in the range of fromabout 540° C. to about 580° C. for about 5 minutes. The sheet may bewound into a coil or unwound into sheet during the thermal processingstep 440.

After the thermal processing step 440, the thickness of the sheet may befurther reduced by a cold rolling step 450. The cold rolling step 450may be substantially similar to the hot rolling step 430 except that thecold rolling step 450 may be carried out at room or slightly elevatedtemperatures. In one embodiment, the cold rolling step 450 may furtherreduce the thickness of the monolithic or composite sheet from about 3.4mm (0.135 inch) to about 1 mm (0.039 inch) translating to a thicknessreduction of approximately 70%. In other embodiments, the cold rollingstep 450 may reduce the thickness of the sheet by about 50% to about60%, or by at least about 80%. In general, the thickness of the sheetmay be reduced accordingly depending on the requirements of theautomotive application. Like above, the sheet may be wound into a coilor unwound into sheet during the cold rolling step 450.

After the cold rolling step 450, the monolithic or composite sheet maybe subjected to a solution heat treatment (SHT) step 460. In oneembodiment, the SHT step 460 includes heating the sheet to a temperaturein the range of from about 540° C. to about 580° C. for about 5 minutes.Furthermore, the SHT step 460 may include pre-aging the sheet afterquenching. For example, after the high temperature treatment, the sheetmay be subjected to a quenching process to a temperature in the range offrom about 60° C. to about 100° C. followed by coiling of the sheet. Thequenching process may be instantaneous and may involve quenching thesheet in air or water or both. In other instances, the quenching processmay take place at room temperature. In one embodiment, after quenchingto room temperature the process may include instantaneously exposing thesheet to a heating device such as infrared heating lamps or inductionheating or an air furnace as the sheet is being coiled or uncoiled,whereby the exposure and subsequent coil cooling over about 1 hour toabout 24 hours or longer may pre-age or alter the microstructure of thesheet. In one embodiment, after SHT, quenching to room temperature andcoiling the coil, the coil can subsequently be heated in a furnace andallowed to cool inside the furnace or outside in ambient temperature.

After the SHT step 460, various tests may be carried out on themonolithic or composite sheet in a testing step 470. In some embodiment,the testing step 470 for characterizing the sheet may include hemperformance, mechanical properties, shelf life, after paint-bakestrength, dent resistance, surface quality, formability, corrosionresistance and grain size, among others.

Reference is now made to FIG. 5 illustrating a process flow diagram 500of the various steps of producing a composite sheet according to oneembodiment of the present disclosure. In one embodiment, a compositeingot having at least two different aluminum alloy composition may beproduced by a roll bonding step 510. For example, a first ingot may beproduced by casting a first aluminum alloy material and a second ingotmay be produced by casting a second aluminum alloy material. The twoingots may be roll bonded to each other by placing one ingot on top ofanother and applying mechanical forces to bring about bonding of theingots. In one embodiment, a composite ingot may be produced bymetallurgically bonding (e.g., lattice structures of the materials areforced into conformance with each other) at least two monolithic ingotsto each other. In some instances, metallurgical bonding may utilize highpressure leading to deformation of the layers. In some embodiments,prior to the roll bonding step 510, each monolithic ingot may behomogenized by heating the ingot at temperatures ranging from about 540°C. to about 570° C. for about 4 hours.

After a composite ingot has been produced by a roll bonding step 510,the ingot may be subjected to hot rolling 530, batch annealing 540, coldrolling 550 and solution heat treatment 560 processes that aresubstantially similar to those described above. And like above, theresulting composite sheet may be evaluated by a testing step 570 toprovide the necessary materials properties and characteristics.

One of the ways of characterizing an automotive panel is hemmingperformance. In general, an automotive panel may be associated with ahem rating based on its hemming performance, where the better thehemmability, the lower the likelihood of the automotive panel to suffersignificant cracking when the automotive panel is bent and/or foldedduring the manufacture of such automotive panel.

Returning now to FIGS. 1-2, an outer panel 210 and an inner panel 230may be hemmed together to produce an automotive panel 110. The hemmingmay result in the formation of a flange by bending and/or folding edgesof each of the two panels 210, 230 together via suitable mechanicaltechniques. The hemming site may be evaluated and the automotive panel110 may be assigned a hem rating. In one embodiment, an outer panel 210and an inner panel 230 may be hemmed together to produce an automotivepanel 110 by rope hem, relieved flat hem or flat hem, which may beconsidered one of the more challenging hemming techniques (e.g., morechallenging than rope hem or relieved flat hem).

Reference is now made to FIG. 6 showing the processing steps for hemmingan automotive panel. In step 602, an outer sheet 720 may be coupled toan inner sheet 740 after each sheet 720, 740 has been pre-strained(e.g., 7%, 11%, 15%). As shown, a portion of the outer sheet 720 may bebent by about 90 degrees with respect to the inner sheet 740. In oneexample, the thickness of the outer sheet 720 may be about 1 mm and thethickness of the inner sheet 740 may be about 1 mm. In another example,the thickness of the outer sheet 720 may be about 0.5 mm and thethickness of the inner sheet 740 may be about 0.5 mm. In other examples,the outer sheet 720 and the inner sheet 740 may include variousthickness combinations. Labels “1 t” and “6 t” mean one time and sixtimes the thickness of the sheet, respectively.

In step 604, additional forces may be applied to continue bending theouter sheet 720 by approximately another 90 degrees with respect to theinner sheet 740 with an overall bending angle of about 180 degrees.Bending of the outer and inner sheets 720, 740 may be accomplished bysuitable mechanical devices. Subsequently, in step 606, a hemming site760 may be formed after the outer sheet 720 has been substantially bentto wrap around a portion of the inner sheet 740.

Reference is now made to FIG. 7 showing cross-sectional views of thevarious hems formed by bending an outer sheet 720 (e.g., outer layer)around an inner sheet 740 (e.g., inner layer) as substantially describedabove. The dimensions and units as shown are in millimeters (mm). Asdiscussed above, the hemming process may occur by bending the outersheet 720 over a portion of the inner sheet 740 by about 180 degreeswith a bending radius R of 1.0 mm (rope hem), 0.75 mm (relieved flathem) and 0.50 mm (flat hem) in accordance with ASTM E290-97A. In theseexamples, the radii are for a 1 mm thick sheet. In one embodiment, thejoining of the outer sheet 720 and the inner sheet 740 may produce acomposite sheet. In another embodiment, the outer sheet 720 may be anouter panel formed of a first composite and the inner sheet 740 may bean outer panel formed of a second composite, the two panels 720, 740capable of being combined to produce an automotive panel such as a hoodor a deck lid.

Reference is now made to FIG. 8 showing flat hem rating standards forside-by-side evaluation and comparison of a hemming site 760 of eachspecimen, and for assigning an associated flat hem rating. A score maybe given to the specimens according to the following flat hem ratingscale as shown in Table 1.

TABLE 1 Flat hem rating scale. 1 No cracking (mild to moderate orangepeel is acceptable) 2 Heavy orange peel 3 Cracks visible with 3Xmagnification 4 Cracks visible with naked eye 5 Fracture or continuouscrack along the bend (e.g., hemming site)

In general, flat hem rating of 1 is the best and flat hem rating of 5 isthe worst. Orange peel is broadly understood as general grain rougheningthat occurs when materials with large grain sizes and/or specificorientations are deformed.

Reference is now made to FIG. 9 showing cross-sectional opticalmicrographs of hemming sites 760. A composite sheet with generallyacceptable flat hem performance (e.g., acceptable flat hem rating) mayexhibit minimal to no cracking on the surface as shown by the opticalmicrograph on the left 920, while a composite sheet with generallyunacceptable flat hem performance (e.g., unacceptable flat hem rating)may exhibit substantial cracking on the surface (as illustrated by thearrows) as shown by the optical micrograph on the right 940.Specifically, generally acceptable optical micrographs 920 can beassociated with flat hem ratings of 1 or 2 while generally unacceptableoptical micrographs 940 can be associated with flat hem ratings of 3, 4or 5.

Returning now to FIG. 3, in some embodiments, each composite sheet 320,360 may be capable of achieving a flat hem rating of not worse than 5,or not worse than 4, or not worse than 3, or not worse than 2, or notworse than 1.

In one embodiment, flat hem rating can be measured longitudinal (e.g., 0degrees, parallel) to the rolling direction of the composite sheet. Therolling direction is the direction in which the composite sheet isrolled through the mechanical rollers (e.g., hot rolling, cold rolling)during the manufacture of such composite sheet. In other instances, flathem ratings may be measured transverse (90 degrees) or diagonal (45degrees) to the rolling direction of the composite sheet. Flat hemratings longitudinal to the rolling direction are generally worse thanflat hem ratings transverse or diagonal to the rolling direction.

In some embodiments, a composite sheet 320, 360 may be pre-strainedprior to flat hem testing. As used herein, “pre-strain” and the likemeans the amount of strain placed on a composite sheet, such as by atensile tester (e.g., Instron tensile test machine). With pre-strain, acomposite sheet may be put to plastic strain beyond the elastic limit ofthe material. In some instances, pre-strain may be reflective of theamount of strain that an automotive panel may be subjected to during themanufacture of such automotive panel.

In some instances, a composite sheet may be pre-strained at about 7%, orat about 11%, or at about 15%, prior to flat hem testing. In otherinstances, the composite sheet may be pre-strained to at least about 1%,or at least about 2%, or at least about 3%, or at least about 4%, or atleast about 5%, or at least about 6%, or at least about 8%, or at leastabout 10%, or at least about 12%, or at least about 14%, or at leastabout 16%. Accordingly, the flat hem rating of a composite sheet may bemeasured after the composite sheet has been pre-strained to suchpre-strain level.

In some embodiments, a composite sheet may be capable of achieving aflat hem rating of not worse than 5, or not worse than 4, or not worsethan 3, or not worse than 2, or not worse than 1, at a pre-strain levelof at least about 1%. In general, composite sheets with better flat hemratings at higher pre-strain levels may be better at forming automotivepanels with complex shapes and configurations.

In some embodiments, the flat hem ratings of a composite sheet may bemeasured at different time periods. As used herein, “time period” andthe like means the amount of time that has elapsed, whether naturally orartificially, after a composite sheet has been produced by completingthe solution heat treatment but prior to flat hem testing. For example,the flat hem rating of a composite sheet may be measured at time periodsof at least about 7 days, or at least about 14 days, after the compositesheet has been produced. In other embodiments, the flat hem rating ofthe composite sheet may be measured at time periods of at least about 30days, or at least about 45 days, or at least about 60 days, or at leastabout 75 days, or at least about 90 days, or other time periods, afterthe composite sheet has been produced.

In some embodiments, a composite sheet may be capable of achieving aflat hem rating of not worse than 5, or not worse than 4, or not worsethan 3, or not worse than 2, or not worse than 1, at a time period of atleast about 7 days. In other embodiments, a composite sheet may becapable of achieving a flat hem rating of not worse than 5, or not worsethan 4, or not worse than 3, or not worse than 2, or not worse than 1,at a time period of at least about 14 days, or at least about 21 days,or at least about 30 days, or at least about 60 days, or at least about90 days.

In general, composite sheets with better flat hem ratings after longertime periods may have better shelf life. In other words, the compositesheet need not be formed into an automotive panel immediately after theproduction of such composite sheet, but may instead remain on the shelffor the measured time period prior to being used to form the automotivepanel. The hemming performance of an automotive panel generallydecreases with increasing shelf life (e.g., flat hem ratings of 1 afterabout 30 days and 3 after at about 90 days). This decrease in flat hemrating may be due to changes in material properties with time.

As used herein, “shelf life” and the like means the length of time(e.g., age, time period) over which a composite sheet continues to meetall applicable specification requirements such as flat hem rating. Forexample, the shelf life of a composite sheet may be associated withnatural aging, which includes changes, if any, to the composite sheetafter exposing the composite sheet to normal environmental conditionsfor a predetermined time period. In one embodiment, shelf life studiesvia natural aging experiments may be carried out by initially measuringthe flat hem rating of a composite sheet at about 30 days afterproduction, and repeating the same measurement at about 90 days afterproduction, where the composite sheet has been exposed to and maintainedat ambient room condition (e.g., sitting on a shelf in a room) duringthe two measurements. In some instances, natural aging may occur to acomposite sheet during, for example, storing of the composite sheetafter production but prior to the composite sheet being shipped to astamping plant, the amount of time spent by the composite sheet in thestamping plant, and storing of the composite sheet after the stampingplant but prior to the stamped composite sheet being shipped to anassembly plant.

In addition to flat hem rating, mechanical properties of a compositesheet, including such properties as tensile yield strength (TYS),ultimate tensile strength (UTS), total and uniform elongation (%), amongothers, may be measured. In some embodiments, the mechanical propertiesmay be measured transverse to the rolling direction of the compositesheet. In other embodiments, the mechanical properties can be determinedlongitudinal or diagonal to the rolling direction of the compositesheet. Mechanical properties transverse to the rolling direction aregenerally worse than mechanical properties longitudinal or diagonal tothe rolling direction.

In some embodiments, a composite sheet according to one embodiment ofthe present disclosure may achieve TYS of at least about 100 MPa, or atleast about 110 MPa, or at least about 120 MPa, or at least about 130MPa, or at least about 140 MPa, or at least about 150 MPa. In otherembodiments, a composite sheet according to one embodiment of thepresent disclosure may achieve UTS of at least about 200 MPa, or atleast about 210 MPa, or at least about 220 MPa, or at least about 230MPa, or at least about 240 MPa, or at least about 250 MPa, or at leastabout 260 MPa, or at least about 270 MPa, or at least about 280 MPa, orat least about 290 MPa, or at least about 300 MPa. In some instances, acomposite sheet according to one embodiment of the present disclosuremay achieve elongations (e.g., total, uniform) of at least about 10%, orat least about 12%, or at least about 14%, or at least about 16%, or atleast about 17%, or at least about 18%, or at least about 19%, or atleast about 20%, or at least about 21%, or at least about 22%, or atleast about 23% , or at least about 24%, or at least about 25%.

For measuring the mechanical properties of a composite sheet, standardtensile test specimens may be machined and tested per ASTM Methods B557and E8. In one embodiment, the standard tensile test specimen may besubstantially similar to a “dog-bone” shaped specimen 1000 as shown inFIG. 10. In one example, length (L) of the specimen 1000 may be in therange of from about 228.6 mm to about 279.4 mm (9 inches to 11 inches),thickness may be not greater than about 12.7 mm (0.5 inch), and widthvarying from about 12.7 mm (0.5 inch) (W1) to about 19.1 mm (0.75 inch)(W2). In other examples, the specimen 900 can come in a variety ofshapes and sizes.

Like flat hem ratings, in some embodiments, mechanical properties of acomposite sheet may be measured at different time periods similar tothose described above. Furthermore, the mechanical properties of acomposite sheet according to the present disclosure may maintainacceptable mechanical characteristics and properties with relativelylong shelf life. In other words, the strength and elongation of thecomposite sheet do not significantly decrease after extended timeperiods or natural aging.

Another way of characterizing an automotive panel is paint bake (PB)strength, which may be indicative of its dent resistance or the abilityof the automotive panel to avoid and/or minimize dents and dings whilein service. In one embodiment, an automotive panel may be subjected to aPB treatment to simulate actual processing condition associated with theautomotive panel. In some instances, the PB treatment may also bereferred to as artificial aging. For example, the automotive panel maybe thermally treated to increase its PB strength and dent resistance. Inthese instances, the outer and inner panels of the automotive panel maybe treated separately or in combination. In general, the greater the PBstrength of a composite sheet after a PB process (e.g., tensile yieldstrength and ultimate tensile strength after paint bake), the greaterthe ability of the composite sheet to withstand dents and dings while inservice. Different automobile manufacturers may have different levels ofminimum PB strength standards for various automotive applications.

In one example, artificial aging includes subjecting a composite sheetto thermal treatment cycles to simulate process conditions. In oneembodiment, a PB cycle may include a combination of pre-straining (e.g.,at 2%) and heating (e.g., at about 170° C. for about 20 minutes). Inother instances, the PB cycle may include treatment cycles at differenttemperatures and/or time periods, with or without pre-straining Thecomposite sheet may subsequently be air cooled to room temperature andits mechanical properties may be tested. In some embodiments, themechanical properties to be tested after a PB cycle include the likes oftensile yield strength (TYS), ultimate tensile strength (UTS) andelongation, among others. In some instances, the TYS and UTS of thecomposite sheet after a PB cycle may be referred to as after PBstrength.

In some embodiments, a composite sheet according to one embodiment ofthe present disclosure may achieve after PB strength of at least about150 MPa, or at least about 180 MPa, or at least about 190 MPa, or atleast about 200 MPa, or at least about 220 MPa, or at least about 240MPa, or at least about 260 MPa, or at least about 280 MPa, or at leastabout 300 MPa, or at least about 310 MPa, or at least about 320 MPa, orat least about 330 MPa, or at least about 340 MPa, or at least about 350MPa. In other embodiments, a composite sheet according to one embodimentof the present disclosure may achieve elongations (e.g., total,uniform), after a PB cycle, of at least about 10%, or at least about12%, or at least about 14%, or at least about 16%, or at least about 17%, or at least about 18%, or at least about 19%, or at least about 20%,or at least about 21%, or at least about 22%, or at least about 23%, orat least about 24%, or at least about 25%.

As discussed above, natural aging includes maintaining a composite sheetat room temperature for a desired time period or duration. In someinstances, natural aging may occur to a material causing changes inperformance (e.g., flat hem ratings) or mechanical properties (e.g.,strength of material before and after thermal treatment), among otherproperties. For instance, 6xxx series aluminum alloys may haverelatively short shelf life as the materials tend to naturally age withtime. In other words, the flat hem performance a 6xxx series aluminumalloy may decrease with increasing shelf life. Thus, upon hemming of the6xxx series aluminum alloy into an automotive panel, cracks or fracturesand the like, may result.

Another way of characterizing an automotive panel is limiting domeheight, which may be used to assess the formability of the automotivepanel. As used herein, “limiting dome height” refers to a maximum heightof a dome formed by a composite sheet for assessing, at least in part,the formability of the composite sheet. In one embodiment, limiting domeheight may be determined by rigidly clamping and stretching a compositesheet to the point of plastic instability (e.g., fracture) using ahemispherical-shaped structure such as a dome. In one example, thestretching may be carried out by mechanical force. The point at whichthe composite sheet fractures defines the limiting dome height and themaximum load that the composite sheet may sustain. In general, thegreater the limiting dome height, the better the formability of thematerial.

As used herein, “formability” and the like means the relative ease withwhich a composite sheet can be shaped through plastic deformation. Forexample, the formability of an automotive panel fabricated of acomposite sheet may be determined, at least in part, by the limitingdome height and in some instances, elongation (higher elongationpercentages may indicate better formability) of the composite sheet,among other properties. In general, the better the formability of thecomposite sheet, the easier it is to manipulate the composite sheet intoa desired shape. The extent to which the composite sheet can bestretched before failure occurs may also be known as the formability orforming limit.

In some embodiments, the composite sheet may achieve a limiting domeheight of at least about 5 mm, or at least about 10 mm, or at leastabout 15 mm, or at least about 20 mm, or at least about 21 mm, or atleast about 22 mm, or at least about 23 mm, or at least about 24 mm, orat least about 25 mm, or at least about 26 mm, or at least about 27 mm,or at least about 28 mm, or at least about 29 mm, or at least about 30mm.

In some embodiments, the formability of an automotive panel may beinfluenced by the strain-hardening coefficient (n) and thewidth-to-thickness strain ratio (R). The strain-hardening coefficient(n) and the width-to-thickness strain ratio (R) are dimensionlessconstants used to measure a material's formability, where the larger thevalue of the strain-hardening coefficient (n) and the width-to-thicknessstrain ratio (R), the better the formability of the material. Inaddition, higher n and R values may indicate better resistance againstthinning, wrinkling and other artifacts.

In some embodiments, a composite sheet according to one embodiment ofthe present disclosure may achieve smaller grain sizes, which mayenhance the composite sheet's formability, hemming performance andsurface appearance or quality, among other attributes.

Reference is now made to FIG. 11 illustrating a process flow diagram1100 of the various steps of manufacturing a composite sheet accordingto one embodiment of the present disclosure. For example, a method ofmanufacturing a composite sheet includes producing an Al—Mg—Si alloy1110. In some instances, the Al—Mg—Si alloy may be a 6xxx seriesaluminum alloy and can be produced by at least one of roll bonding,multi-alloy casting and direct-chill casting as described herein, amongother techniques. Subsequently, the method includes producing a firstAl—Mg alloy 1120, which can be a 3xxx series aluminum alloy. Like theAl—Mg—Si alloy, the Al—Mg alloy can be produced by at least one of rollbonding, multi-alloy casting and direct-chill casting as describedherein, among other techniques. Although the process flow shows theAl—Mg—Si alloy being produced ahead of the Al—Mg alloy, it will beappreciated that the Al—Mg alloy can be produced ahead of the Al—Mg—Si.Alternatively, the two alloys may be produced concomitantly orsimultaneously.

In one embodiment, the method includes placing a first surface of theAl—Mg—Si alloy in physical contact with a first surface of the firstAl—Mg alloy 1130. The placing step 1130 results in producing a bi-layercomposite sheet 1140, which can achieve a flat hem rating of not worsethan 3, or not worse than 2, or not worse than 1.

Additionally, the method of manufacturing a composite sheet furtherincludes producing a second Al—Mg alloy step 1150. The method ofproducing the second Al—Mg alloy can be substantially similar in allrespect to the first Al—Mg alloy with the exception that the alloys neednot have the same chemical composition and/or thickness. Next, themethod may include a second placing step 1160 whereby a second surfaceof the Al—Mg—Si alloy can be placed in contact with a first surface ofthe second Al—Mg alloy. In one embodiment, the first and second surfacesof the Al—Mg—Si alloy are opposite one another. The placing step 1160results in producing a tri-layer composite sheet 1170, which like thebi-layer composite sheet, can achieve a flat hem rating of not worsethan 3, or not worse than 2, or not worse than 1.

The following examples demonstrate the feasibility of a multi-alloycomposite sheet as an automotive panel.

EXAMPLE 1 Tri-Layer Composite with AA3104 and AA6013 Aluminum Alloys

A tri-layer composite sheet 360 having a cross-section substantiallysimilar to that shown in FIG. 3 can be produced by casting using theprocess flow as shown in FIG. 4. Specifically, the tri-layer compositesheet 360 can be produced by multi-alloy casting an AA3104 Al—Mg alloyfirst skin layer 370, an AA6013 Al—Mg—Si alloy core layer 380, andanother AA3104 Al—Mg alloy as second skin layer 390. The AA3104 andAA6013 aluminum alloys have the chemical composition (in weightpercentages) as shown in Table 2.

TABLE 2 Chemical composition (in wt. %) of AA3104 and AA6103 aluminumalloys. Alloy Si Fe Cu Mn Mg Al AA3104 0.22-0.30 0.52-0.58 0.15-0.200.93-0.97 1.17-1.26 Remainder AA6013 0.65-0.75 0.25-0.29 0.85-1.040.30-0.32 0.90-1.04 Remainder

The resulting composite ingot of AA3104 and AA6013 aluminum alloys has awidth of about 0.4 meter (16 inches), a length of about 1.4 meters (55inches), and a thickness of about 0.3 meter (12 inches), and can behomogenized at about 560° C. for about 4 hours. Hot rolling of thecomposite ingot results in the production of a composite sheet having athickness of about 3.4 mm. A first sample (Example 1A) is subjected tobatch annealing at a temperature of about 425° C. for about 60 minuteswhile a second sample (Example 1B) is subjected to solution heattreatment at a temperature of about 570° C. for about 5 minutes. Thethickness of both samples are further reduced by cold rolling to T4condition with a total thickness T2 of about 1 mm. The thickness of theAA3104 first skin layer 370 is about 25% (0.25 mm) of the totalthickness T2 of the composite sheet 360, the thickness of the AA6013core layer 380 is about 65% (0.65 mm) of the total thickness T2 of thecomposite sheet 360, and the thickness of the AA3104 second skin layer390 is about 10% (0.10 mm) of the total thickness T2 of the compositesheet 360. Both samples are further subjected to a solution heattreatment process at a temperature of about 570° C. for about 5 minutes.

The tri-layer composite sheets 360 are evaluated against a controlsample of a monolithic sheet of AA6022 Al—Mg—Si alloy, which can beproduced by direct chill casting using the process flow as shown in FIG.4. Specifically, the AA6022 ingot can be homogenized at about 550° C.for about 4 hours, followed by hot rolling to a thickness of about 3.4mm. The resulting AA6022 monolithic sheet is subsequently batch annealedat a temperature of about 425° C. for about 60 minutes, and cold rolledto T4 condition to a thickness of about 1 mm. The AA6022 control sampleis further solution heat treated at a temperature of about 550° C. forabout 5 minutes.

The material properties and performance of the tri-layer compositesheets 360 and the AA6022 control sample are shown in Table 3. Allmeasurements are tested after 30 days of natural aging and in theorientation transverse to the rolling direction with the exception ofthe flat hem rating, which is tested in the orientation longitudinal tothe rolling direction. The flat hem ratings are measured at threedifferent levels of pre-strain (7%, 11%, 15%). Mechanical propertiesincluding tensile yield strength (TYS), ultimate tensile strength (UTS)and elongation (%) designated by a dagger symbol (†) are measured aftera paint bake cycle consisting of pre-straining the sheet at 2% andheating at about 170° C. for about 20 minutes.

TABLE 3 Performance of the tri-layer AA3104/AA6013/AA3104 compositesheet. Material Property 6022-1 Example 1A Example 1B Thermal ProcessingAnneal Anneal SHT Flat Hem at 7%/11%/15% 3/3/4 1/1/1 1/1/1 TYS (MPa)123.4 115.1 117.2 UTS (MPa) 231.7 246.1 246.8 Total Elongation (%) 24.218.6 21.3 Uniform Elongation (%) 21.8 18.5 20.3 TYS^(†) (MPa) 231.0195.8 197.9 UTS^(†) (MPa) 297.2 278.5 282.0 Elongation^(†) (%) 20.5 20.318.0 n Value 0.259 0.267 0.262 R Value 0.637 0.663 0.641

As shown in Table 3, the hemming performance of the tri-layer compositesheets 360 (Examples 1A and 1B) are superior to the AA6022 controlsample. Specifically, both tri-layer composite sheets 360 are able tosustain good flat hem ratings across all three pre-strain levels (e.g.,1's vs. 3's and 4's at 7%, 11% and 15% pre-strain).

In addition, the mechanical properties (e.g., TYS, UTS and elongation)of the tri-layer composite sheets 360 (Examples 1A and 1B) aresubstantially comparable to those of the AA6022 control sample.Specifically, tensile yield strength (e.g., average 116 MPa vs. 123MPa), ultimate tensile strength (e.g., average 246 MPa vs. 232 MPa), andelongation (e.g., average 20% vs. average 23%) are substantially similarbetween the tri-layer composite sheets 360 and the AA6022 controlsample.

Furthermore, the tri-layer composite sheets 360 are able to maintain themechanical performance after a paint-bake treatment. Specifically,tensile yield strength (e.g., average 197 MPa vs. 231 MPa), ultimatetensile strength (e.g., average 280 MPa vs. 297 MPa), and elongation(e.g., average 19% vs. 21%), after the paint-bake cycle, aresubstantially comparable between the tri-layer composite sheets 360 andthe AA6022 control sample. In general, each tri-layer composite sheet360 meets a minimum after PB strength of at least about 190 MPa.

The tri-layer composite sheets 360 can also achieve comparable if notenhanced formability as automotive panels relative to the AA6022 controlsample. Specifically, the tri-layer composite sheets 360 have similar ifnot slightly better n (e.g., average 0.265 vs. 0.259) and R values(e.g., average 0.652 vs. 0.637) in comparison to the AA6022 controlsample.

The material properties and performance of the tri-layer compositesheets 360 and the AA6022 control sample, after 3 months of naturalaging, are shown in Table 4. The measurements are similar in all respectto those of Table 3 with the exception of the time period.

TABLE 4 Performance of the tri-layer composite sheets after 3 monthsnatural aging. Material Property 6022-1 Example 1A Example 1B ThermalProcessing Anneal Anneal SHT Flat Hem at 7%/11%/15% 2/3/4 1/2/2 1/1/1TYS (MPa) 141.3 117.9 117.2 UTS (MPa) 249.6 255.1 246.8 Total Elongation(%) 21.8 24.1 19.5 Uniform Elongation (%) 20.9 20.9 19.4 TYS^(†) (MPa)231.0 194.4 192.4 UTS^(†) (MPa) 297.9 281.3 276.5 Elongation^(†) (%)20.0 22.0 17.5 n Value 0.238 0.263 0.263 R Value 0.654 0.738 0.544

As shown in Table 4, the hemming performance of the tri-layer compositesheets 360 (Examples 1A and 1B) remain relatively unchanged after 3months of natural aging, and maintains superior performance to theAA6022 control sample. Specifically, both tri-layer composite sheets 360are still able to sustain good flat hem ratings across all threepre-strain levels even after 3 months of natural aging (e.g., 1's and2's at 7%, 11% and 15% pre-strain), and are still better than the AA6022control sample (e.g., 2, 3 and 4 at 7%, 11% and 15% pre-strain,respectively).

In addition, the mechanical properties (e.g., TYS, UTS and elongation)of the tri-layer composite sheets 360 (Examples 1A and 1B) remainsubstantially comparable to those of the AA6022 control sample after 3months. Specifically, tensile yield strength (e.g., average 118 MPa vs.141 MPa), ultimate tensile strength (e.g., average 251 MPa vs. 250 MPa),and elongation (e.g., average 22% vs. average 21%) are substantiallysimilar between the tri-layer composite sheets 360 and the AA6022control sample. Also, the mechanical properties of both tri-layercomposite sheets 360 did not sustain substantial degradation after 3months natural aging (e.g., TYS: 115.1 MPa to 117.9 MPa (Example 1A),117.2 MPa to 117.2 MPa (Example 1B); UTS: 246.1 MPa to 255.1 MPa(Example 1A), 246.8 MPa to 246.8 MPa (Example 1B)).

Furthermore, the tri-layer composite sheets 360 are able to maintain themechanical after paint-bake performance after 3 months. Specifically,the after paint-bake tensile yield strength (e.g., average 193 MPa vs.231 MPa), ultimate tensile strength (e.g., average 279 MPa vs. 298 MPa),and elongation (e.g., average 20% vs. 21%), after 3 months, aresubstantially comparable between the tri-layer composite sheets 360 andthe AA6022 control sample. Like above, the mechanical properties of thetri-layer composite sheets 360 did not sustain substantial degradationafter 3 months natural aging (e.g., TYS: 195.8 MPa to 194.4 MPa (Example1A), 197.9 MPa to 192.4 MPa (Example 1B); UTS: 278.5 MPa to 281.3 MPa(Example 1A), 282.0 MPa to 276.5 MPa (Example 1B)). In general, eachtri-layer composite sheet 360 meets a minimum after PB strength of atleast about 190 MPa.

The tri-layer composite sheets 360 can also achieve comparable if notenhanced formability as automotive panels relative to the AA6022 controlsample after 3 months. Specifically, the tri-layer composite sheets 360have similar if not slightly better n (e.g., average 0.263 vs. 0.238)and R values (e.g., average 0.641 vs. 0.654) in comparison to the AA6022control sample after 3 months. Furthermore, the n and R values did notsustain substantial degradation after 3 months natural aging (e.g., nvalue: 0.267 to 0.263 (Example 1A), 0.262 to 0.263 (Example 1B); Rvalue: 0.663 to 0.738 (Example 1A), 0.641 to 0.544 (Example 1B)).

References is now made to FIGS. 12-13 showing cross-sectional opticalmicrographs of hemming sites of 6022-1 and Example 1A after 3 monthsnatural aging. Specifically, the optical micrographs in FIG. 12 aredirected to the AA6022 control sample while the optical micrographs inFIG. 13 are directed to the tri-layer composite sheet 360 (Example 1A).As shown in FIG. 12, the optical micrograph on the left 1210 is for anAA6022 control sample that has been pre-strained at 7% and has a flathem rating of 2, the optical micrograph in the middle 1220 has beenpre-strained at 11% and has a flat hem rating of 3, and the opticalmicrograph on the right 1230 has been pre-strained at 15% with a flathem rating of 4. Similarly, as shown in FIG. 13, the optical micrographon the left 1310 is for a tri-layer composite sheet 360 (Example 1A)that has been pre-strained at 7% and has a flat hem rating of 1, theoptical micrograph in the middle 1320 has been pre-strained at 11% witha flat hem rating of 2, and the optical micrograph on the right 1330 hasbeen pre-strained at 15% with a flat hem rating of 2.

In short, the flat hem ratings of the tri-layer composite 360 are betterthan the AA6022 control sample at each pre-strain level (e.g., 7%, 11%and 15%) where no cracks are visible. In contrast, other than sample1210 at the 7% pre-strain, both 11% and 15% pre-strain samples 1220,1230 showed cracking (as illustrated by the arrows) on the surfaces ofthe hemming sites. Furthermore, the tri-layer composite 360 is able tomaintain minimal to nearly zero cracking across the three differentpre-strain levels 1310, 1320, 1330 (e.g., from 7% to 11% to 15%) assubstantially shown in FIG. 13, in contrast to the incremental crackingexperienced by the AA6022 control samples 1210, 1220, 1230 (e.g., from7% to 11% to 15%, zero cracks to one crack to two cracks), thusconfirming the tri-layer composite sheets 360 having better flat hemratings than the AA6022 control sample.

EXAMPLE 2 Bi-Layer Composite with AA3104 and AA6013 Aluminum Alloys

A bi-layer composite sheet 320 having a cross-section substantiallysimilar to that shown in FIG. 3 can be produced by casting using theprocess flow as shown in FIG. 4. Specifically, the bi-layer compositesheet 320 can be produced by multi-alloy casting an AA3104 Al—Mg alloyskin layer 330 and an AA6013 Al—Mg—Si alloy core layer 340, and placingthe two layers 330, 340 in physical contact with each other. The AA3104and AA6013 aluminum alloys have the chemical composition (in weightpercentages) as shown in Table 5.

TABLE 5 Chemical composition (in wt. %) of AA3104 and AA6103 aluminumalloys. Alloy Si Fe Cu Mn Mg Al AA3104 0.22-0.24 0.52-0.57 0.15-0.200.93-1.01 1.17-1.22 Remainder AA6013 0.65-0.71 0.20-0.30 0.85-0.910.30-0.32 0.90-0.98 Remainder

Like above, the resulting composite ingot of AA3104 and AA6013 aluminumalloys has a width of about 0.4 meter (16 inches), a length of about 1.4meters (55 inches), and a thickness of about 0.3 meter (12 inches), andcan be homogenized at about 560° C. for about 4 hours. Hot rolling ofthe composite ingot results in the production of a composite sheethaving a thickness of about 3.4 mm. A first sample (Example 2A) issubjected to batch annealing at a temperature of about 425° C. for about60 minutes while a second sample (Example 2B) is subjected to solutionheat treatment at a temperature of about 570° C. for about 5 minutes.The thicknesses of both samples are further reduced by cold rolling toT4 condition with a total thickness Ti of about 1 mm.

The thickness of the AA3104 skin layer 330 is about 25% (0.25 mm) of thetotal thickness T1 of the composite sheet 320 while the thickness of theAA6013 core layer 340 is about 75% (0.75 mm) of the total thickness T1of the composite sheet 320. Both samples are further subjected to asolution heat treatment process at a temperature of about 570° C. forabout 5 minutes. Like above, the bi-layer composite sheets 320 areevaluated against a control sample of a monolithic sheet of AA6022Al—Mg—Si alloy, which can be produced by the method described above.

The material properties and performance of the bi-layer composite sheets320 and the AA6022 control sample are shown in Table 6, where themeasurements are similar to those described above.

TABLE 6 Performance of the bi-layer AA3104/AA6013 composite sheet.Material Property 6022-2 Example 2A Example 2B Thermal Processing AnnealAnneal SHT Flat Hem at 7%/11%/15% 3/3/4 3/3/4 1/1/2 TYS (MPa) 123.4146.2 148.9 UTS (MPa) 231.7 289.6 300.6 Total Elongation (%) 24.2 21.325.2 Uniform Elongation (%) 21.8 19.6 21.2 TYS^(†) (MPa) 231.0 252.3258.6 UTS^(†) (MPa) 297.2 339.2 349.6 Elongation^(†) (%) 20.5 18.0 20.5n Value 0.259 0.260 0.262 R Value 0.637 0.610 0.576

As shown in Table 6, although the hemming performance of one of thebi-layer composite sheets 320 (Example 2A) is comparable to that of theAA6022 control sample (e.g., 3's and 4's), the hemming performance ofthe other bi-layer composite sheet 320 (Example 2B) has demonstratedsuperior flat hem ratings (e.g., 1's and 2's vs. 3's and 4's) comparedto the AA6022 control sample and across all pre-strain levels (e.g., 7%,11% and 15%).

In addition, the mechanical properties (e.g., TYS, UTS and elongation)of the bi-layer composite sheets 320 (Examples 2A and 2B) aresubstantially better than those of the AA6022 control sample.Specifically, the tensile yield strength (e.g., average 148 MPa vs. 123MPa) and the ultimate tensile strength (e.g., average 295 MPa vs. 232MPa) of the bi-layer composite sheets 320 have achieved improvements ofat least about 15% and at least about 30%, respectively, versus theAA6022 control sample. The bi-layer composite sheets 320 are furtherable to maintain comparable elongation (e.g., average 22% vs. average23%) versus the AA6022 control sample. In general, each bi-layercomposite sheet 320 meets a minimum after PB strength of at least about190 MPa.

Furthermore, the bi-layer composite sheets 320 are able to demonstrateimproved mechanical performance after a paint-bake treatment.Specifically, tensile yield strength (e.g., average 255 MPa vs. 231MPa), ultimate tensile strength (e.g., average 344 MPa vs. 297 MPa), andelongation (e.g., average 19% vs. 20%), after the paint-bake cycle, aresubstantially comparable (and in some instances slightly better) betweenthe bi-layer composite sheets 320 and the AA6022 control sample.

The bi-layer composite sheets 320 can also achieve comparableformability as automotive panels relative to the AA6022 control sample.Specifically, the bi-layer composite sheets 320 have substantiallysimilar n (e.g., average 0.261 vs. 0.259) and R values (e.g., average0.593 vs. 0.637) in comparison to the AA6022 control sample.

The material properties and performance of the bi-layer composite sheets320 and the AA6022 control sample, after 3 months of natural aging, areshown in Table 7. The measurements are similar in all respect to thoseof Table 6 with the exception of the time period.

TABLE 7 Performance of the bi-layer composite sheets after 3 months ofnatural aging. Material Property 6022-2 Example 2A Example 2B ThermalProcessing Anneal Anneal SHT Flat Hem at 7%/11%/15% 2/3/4 2/2/2 2/2/3TYS (MPa) 141.3 151.7 152.4 UTS (MPa) 249.6 295.8 303.4 Total Elongation(%) 21.8 18.4 24.1 Uniform Elongation (%) 20.9 18.2 18.3 TYS^(†) (MPa)231.0 257.2 259.2 UTS^(†) (MPa) 297.9 344.0 348.2 Elongation^(†) (%)20.0 19.0 20.0 n Value 0.238 0.258 0.261 R Value 0.654 0.557 0.570

As shown in Table 7, the hemming performance of the bi-layer compositesheets 320 (Examples 2A and 2B) remain relatively unchanged after 3months of natural aging, and maintains superior performance to theAA6022 control sample. Specifically, both bi-layer composite sheets 320are still able to sustain good flat hem ratings across all threepre-strain levels even after 3 months of natural aging (e.g., 2's and3's at 7%, 11% and 15% pre-strain), and are still better than the AA6022control sample (e.g., 2, 3 and 4 at 7%, 11% and 15% pre-strain,respectively).

In addition, the mechanical properties (e.g., TYS, UTS and elongation)of the bi-layer composite sheets 320 (Examples 2A and 2B) remainsuperior than those of the AA6022 control sample after 3 months.Specifically, tensile yield strength (e.g., average 152 MPa vs. 141MPa), ultimate tensile strength (e.g., average 300 MPa vs. 250 MPa), andelongation (e.g., average 20% vs. average 21%) of the bi-layer compositesheets 320 are substantially better than the AA6022 control sample.Also, the mechanical properties of both bi-layer composite sheets 320did not sustain substantial degradation after 3 months natural aging(e.g., TYS: 146.2 MPa to 151.7 MPa (Example 2A), 148.9 MPa to 152.4 MPa(Example 2B); UTS: 289.6 MPa to 295.8 MPa (Example 2A), 300.6 MPa to303.4 MPa (Example 2B)).

Furthermore, the bi-layer composite sheets 320 are able to maintainsuperior mechanical after paint-bake performance after 3 months.Specifically, the after paint-bake tensile yield strength (e.g., average258 MPa vs. 231 MPa), ultimate tensile strength (e.g., average 346 MPavs. 298 MPa), and elongation (e.g., average 20% vs. 20%), after 3months, of the bi-layer composite sheets 320 are better than the AA6022control sample. Like above, the mechanical properties of the bi-layercomposite sheets 320 did not sustain substantial degradation after 3months natural aging (e.g., TYS: 252.3 MPa to 257.2 MPa (Example 2A),258.6 MPa to 259.2 MPa (Example 2B); UTS: 339.2 MPa to 344.0 MPa(Example 2A), 349.6 MPa to 348.2 MPa (Example 2B)). In general, eachbi-layer composite sheet 320 meets a minimum after PB strength of atleast about 190 MPa.

The bi-layer composite sheets 320 can also achieve comparable if notenhanced formability as automotive panels relative to the AA6022 controlsample after 3 months. Specifically, the bi-layer composite sheets 320have similar if not slightly better n (e.g., average 0.260 vs. 0.238)and R values (e.g., average 0.564 vs. 0.654) in comparison to the AA6022control sample after 3 months. Furthermore, the n and R values did notsustain substantial degradation after 3 months natural aging (e.g., nvalue: 0.260 to 0.258 (Example 2A), 0.262 to 0.261 (Example 2B); Rvalue: 0.610 to 0.557 (Example 2A), 0.576 to 0.570 (Example 2B)).

EXAMPLE 3 Bilayer Composite with AA3003 and AA6013 Aluminum Alloys

A bi-layer composite sheet 320 having a cross-section substantiallysimilar to that shown in FIG. 3 can be produced by bonding using theprocess flow as shown in FIG. 5. Specifically, the bi-layer compositesheet 320 can be produced by roll bonding an AA3003 Al—Mg alloy skinlayer 330 to an AA6013 Al—Mg—Si alloy core layer 340. The AA3003 andAA6013 aluminum alloys have the chemical composition (in weightpercentages) as shown in Table 8.

TABLE 8 Chemical composition (in wt. %) of AA3003 and AA6103 alloys.Alloy Si Fe Cu Mn Mg Cr Ti Al AA3003 0.20 0.49 0.11 1.10 0.01 0.006 0.03Remainder AA6013 0.63 0.27 0.85 0.30 0.87 0.033 0.02 Remainder

Like above, the resulting composite ingot of AA3003 and AA6013 aluminumalloys can be hot rolled to a thickness of about 3.4 mm, batch annealedat a temperature of about 425° C. for about 60 minutes, and furtherreduced by cold rolling to T4 condition to a total thickness T1 of about1 mm. The thickness of the AA3003 skin layer 330 is about 20% (0.20 mm)of the total thickness T1 of the composite sheet 320 while the thicknessof the AA6013 core layer 340 is about 80% (0.80 mm) of the totalthickness T1 of the composite sheet 320. The bi-layer composite sheet320 is further subjected to a solution heat treatment process at atemperature of about 570° C. for about 5 minutes. Like above, thebi-layer composite sheet 320 can be evaluated against a control sampleof a monolithic sheet of AA6022 Al—Mg—Si alloy, which can be produced bythe method described above.

The material properties and performance of the bi-layer composite sheets320 and the AA6022 control sample are shown in Table 9, where themeasurements are similar to those described above.

TABLE 9 Performance of the bi-layer AA3003/AA6013 composite sheet.Material 6022-3 Example 3 Flat Hem at 7%/11%/15% 2/3/4 1/1/1 TYS (MPa)106.2 113.8 UTS (MPa) 222.7 244.8 Total Elongation (%) 25.1 25.5 UniformElongation (%) 22.2 20.6 TYS^(†) (MPa) 199.0 198.0 Limiting Dome Height(mm) 25.0 24.1 n Value 0.276 0.271 R Value 0.713 0.676

As shown in Table 9, the hemming performance of the bi-layer compositesheet 320 is superior to the AA6022 control sample. Specifically, thebi-layer composite sheet 320 is able to sustain good flat hem ratingsacross all three pre-strain levels (e.g., 1's vs. 2-4 at 7%, 11% and 15%pre-strain).

In addition, the mechanical properties (e.g., TYS, UTS and elongation)of the bi-layer composite sheet 320 are substantially better than thoseof the AA6022 control sample. Specifically, tensile yield strength(e.g., 114 MPa vs. 106 MPa), ultimate tensile strength (e.g., average245 MPa vs. 223 MPa), and elongation (e.g., average 23% vs. average 24%)of the bi-layer composite sheet 320 are better than the AA6022 controlsample. Furthermore, the bi-layer composite sheet 320 is able todemonstrate comparable mechanical performance to that of the AA6022control sample (e.g., 198 MPa vs. 199 MPa) after a paint-bake treatment.

The bi-layer composite sheet 320 can also achieve comparable formabilityas automotive panels relative to the AA6022 control sample.Specifically, the bi-layer composite sheet 320 has substantially similarlimiting dome height (e.g., 24.1 mm vs. 25.0 mm), and similar n (e.g.,0.271 vs. 0.276) and R values (e.g., 0.676 vs. 0.713) in comparison tothe AA6022 control sample.

The presently disclosed composite sheets may satisfy the needs ofautomotive manufacturers for closure panels, including the likes of ahood, decklid or a door. The composite sheet may be formed ofmulti-alloys capable of achieving improved formability and greater dentresistance after thermal exposure (paint bake), among other propertiesand characteristics. As such, the composite sheet may satisfy theforming and strength requirements for exterior body panel as well asother structural applications. Furthermore, the composite sheet may beartificially aged to increase its strength for higher dent resistance ofthe formed part similar to or better than traditionally used alloys.

Although the multi-alloy composite sheets and methods of manufacturingthe same have been described in detail with reference to severalembodiments, additional variations and modifications exist within thescope and spirit of the disclosure.

What is claimed is:
 1. A method comprising: (a) hot rolling an ingot to form a sheet; (b) after step (a), solution heat treating the sheet; (c) after step (b), cold rolling the sheet; and (d) after step (c), solution heat treating the sheet.
 2. The method of claim 1, wherein step (a) comprises hot rolling the ingot at a temperature of 500 degrees Celsius to 550 degrees Celsius.
 3. The method of claim 2, wherein step (a) comprises a ten-fold reduction in a thickness of the ingot.
 4. The method of claim 1, wherein step (b) comprises solution heat treating the sheet at a temperature of 540 degrees Celsius to 580 degrees Celsius.
 5. The method of claim 1, wherein step (c) comprises cold rolling the sheet at room temperature.
 6. The method of claim 1, wherein step (c) comprises cold rolling the sheet sufficiently to result in an 50% to 80% reduction in a thickness of the sheet.
 7. The method of claim 1, wherein step (d) comprises solution heat treating at a temperature of 540 degrees Celsius to 580 degrees Celsius.
 8. The method of claim 1, wherein step (d) comprises quenching the sheet.
 9. The method of claim 8, wherein step (d) comprises pre-aging the sheet after quenching the sheet.
 10. The method of claim 1, wherein the ingot of step (a) comprises at least one aluminum alloy.
 11. The method of claim 10, wherein the ingot of step(a) comprises a monolithic ingot.
 12. The method of claim 10, wherein the ingot of step(a) comprises a composite ingot.
 13. The method of claim 12, wherein the composite ingot of step (a) comprises a 6xxx series aluminum alloy and a 3xxx series aluminum alloy.
 14. A method comprising: (a) casting an ingot having at least one aluminum alloy; (b) after step (a), hot rolling the ingot to form a sheet; (c) after step (b), solution heat treating the sheet; (d) after step (c), cold rolling the sheet; and (e) after the step (d), solution heat treating the sheet.
 15. The method of claim 14, wherein step (a) comprises multi-alloy casting and/or direct chill casting an ingot having at least one aluminum alloy.
 16. The method of claim 14, wherein step (a) comprises casting a monolithic ingot.
 17. The method of claim 14, wherein step (a) comprises casting a composite ingot.
 18. The method of claim 17, wherein the composite ingot of step (a) comprises a 6xxx series aluminum alloy and a 3xxx series aluminum alloy.
 19. The method of claim 14, wherein step (a) comprises homogenizing the ingot after casting the ingot.
 20. The method of claim 19, wherein step (a) comprises homogenizing the ingot at a temperature of 540 degrees Celsius to 570 degrees Celsius after casting the ingot.
 21. The method of claim 20, wherein step (a) comprises homogenizing the ingot for at least 4 hours after casting the ingot. 